Flame detectors are often used to detect the presence of a flame at installations where flammable and combustible materials are used, stored or dispensed. Early flame detection at these installations can prevent substantial installation damage and/or loss of life by allowing the flames to be quickly extinguished before they are able to spread. In this regard, optical sensors can be employed to detect certain characteristic light frequencies that are emitted by the flames. Oftentimes, these installations are outdoors and can be relatively large in terms of the total area that must be monitored.
The outdoor monitoring of flames can present certain difficulties. For one, flame sensors are relatively sensitive. They can be fragile, and in all instances they must be protected from the elements including rain, cold, ice, snow, and frost. In addition, the sensors should be masked from extraneous ultra-violet (UV) sources that could cause a false alarm. Moreover, when more than one sensor is used to monitor a target area, the UV sensors themselves can be a source of false alarms. More specifically, the detection mechanism for certain types of UV sensors can cause UV light to be emitted by that sensor. For example, if a UV sensor is falsely triggered by a cosmic ray or some other non-flame source, UV light may be emitted by the triggered sensor and the emitted UV light may be detected by another sensor. This other sensor, in turn, could then emit UV light that feeds back to the original falsely triggered sensor. This positive feedback (i.e. self-feeding) effect may then continue until the flame sensor system mistakenly identifies the event as a fire.
Other sources of non-flame UV, including lightning and other electrical arc discharges such as arc welding and electrical motors, can also cause false readings. Stated another way, conventional flame sensors often react the same way to both non-flame related UV and flame related UV. Heretofore, the typical method for suppressing false alarms from lightning was to set the electronic monitor with a delay that would sound an alarm only when a signal from the UV sensor is received for a longer period than a lightning event, e.g. 10 seconds. This period, however, can be too long when protecting highly volatile material.
Another factor that must be considered when detecting flames outdoors involves the relative long distances that are associated with outdoor monitoring. Conventional UV detectors have limited range, and heretofore, the only solution to this deficiency was the costly approach of adding more sensors to the monitoring system to thereby ensure that the entire area of interest was properly covered.
In light of the above, it is an object of the present invention to provide an outdoor flame detector that is protected in its operational environment from adverse factors that include severe thermal, structural and hydrological conditions. Another object of the present invention is to provide an outdoor flame detector that is capable of distinguishing between an actual open flame and a non-flame event, such as an electric arc discharge (e.g. lightening, electric motors and arc welding). Still another object of the present invention is to provide a flame detector that is easy to use, relatively simple to implement and comparatively cost effective.